Telecommunications trials study Flashcards
3 Guided media types
- Twisted pair cable
- Coaxial
- Fibre optical
Twisted Pair Cables
Cheapest, shortest range, greatest EM/noise interference
Twisting reduces crosstalk
4 pairs of wire with different twist lengths
USB cables
Provides power and supports data transfer
- red and black for power
- green and white for signal
Coaxial Cable
Ethernet cables
Twisted pair cable
Coaxial Cable
Has 4 layers:
1. outer plastic sheath
2. Woven copper shield
3. Inner dielectric layer (insulator)
4. Copper core (or copper-clad steel)
Better noise resistance
Skin effect
AC power flows mainly along the surface
- skin depth dereases as frequeny increases
Coaxial cables opeerate at a
Copper-clad steel (CCS)
Increases skin depth
Fibre Optic Cables, pros and cons
Transmits light pulses
Several 100x faster the coaxial (up to 10km)
More immunity to noise, low signal attenutation, not susceptible to EMI, cannot be tapped (Security)
ALSO: digital signals, normally encoded
BUT fragile, expensive
Unguided media and types
NOT physically connected e.g.
radio (broadcast radio, TV)
microwave (directional transmissions e.g. mobile phone networks and satellites)
Infrared (remote controls, short range)
Directionality
Point-to-point: direct
vs
Broadcasting: various receivers
Mircophones/speakers
- Diaphragm vibrates when sound hits it
- Coil of wire with a magnet inside: Faraday’s law of induction
Antenna
Long piece of metal with a running current
- creates a magnetic field
- makes EM waves
- propagates in 3 dimensions
4 Types of radio waves
Direct: line-of-sight
Wave reflected from ground/ground waves: affected by terrain and vegetation (e.g. mountains): due to diffraction
Sky wave: skip propagation (bounces off ionosphere) - long distances only
Amplitude vs Digital signals
Analogues are infinitely variable (VERY complex), digitals have set levels (‘steps’) e.g. binary only has 1 or 0
Digitising steps
- Sampled at regular intervals
- This is quanitsed e.g. rounded to closest digital level (Quantisation)
- Encoded as a binary signal before transmission.
SQE: Stupid Quokkas Eat
Why digitise?
Security, can send more information in digital (channel capacity utilisation - multiplexing), noise immunity (ability to REGENERATE), lower cost for equipment
Bandwidth
Capacity for a channel to convey info (measured in bits for digital transmission)
Attenuation
Decrease in signal intensity (decibels dB)
Signal-to-noise ratio (SNR)
Higher means more resiliant
Regenerative repeaters
Take signal, then amplify: increases range
- first regenerates it to remove the noise
- ONLY for digital
Modulation, why it is important
Taking a message (baseband signal) and modifying a property so the message is easier transmitted.
Has high frequency carrier signal
MODEM: does the modulation
WHY?
1. Reduce antenna size
2. Reduce interference (send things at different frequencies)
3. Allow Multiplexing
3 Analogue Continuous wave modulation types (shift keying)
- Amplitude (very affected by noise, inefficient power use)
- Frequency (less noise interference as noise impacts amplitude, needs high bandwidth)
- Phase (ok for digital, not for analogue)
LESS IMPORTANTL Pulse analogue modulation
Used for brightness of a lightbulb, etc.
- PAM: amplitude matches samples amplitude
- PWM: checks the width as well: Motor and LED control
PPM: position matches sampled
Digital Modulation (transmission of signals in binary)
SHIFT KEYING
1. ASK: amplifude (high for 1, low for 0)
2. FSK: frequency (high for 1, low for 0)
3. PSK Phase (original is 1, shifted 180 out of phase is 0) - appears to flip
Demodulation
Opposite of modulation: getting rid of carrier
1. Antenna is tuned to pick up a specific frequency (by coil and variable capacitor)
2. Diode rectifies the signal (makes positive)
3. Fixed capacitor smooths it
4. Earphones vibrates according to that -> converts to sound.
Diode
Rectifies AC signal (only leaves positive)
Guided media + pros and cons
Guided: transmission media that carries signals with a conductor e.g. fibre optic cables: has a PHYSICAL LINK
- short distance, but expensive for long distance/weird terrain
Power Cables
- Low voltage - copper with PVC insulation
- High voltage - Cu or Al, shielded with polyethylene
- Extra high voltage - Al with steel core, not insulated.
(wire is often stranded to increase flexibility)
Conductor Properies
Cu, Al Gold
High conductivity/low resistivity
High ductility
High Tensile strength
Good corrosion resistance
Low cost
Copper propeties
High tensile strength
good conductivity
good ductility
Pure copper types
Why polymers?
Cheap
Ductile -> needs to be extruded
Low electrical conductivity (insulating)
Flexible
Main polymers
Polyethylene
Polyvinyl Chloride (PVC)
Polypropylene
Nylon
ABS
Polycarbonate
Polyethylene properties
retains insulation in humidity
BUT low softening temperature
Allows water vapour penetration
Relatively expensive
used for high-frequency interior cables (not used outside)
PVC
Not as good an insulates and polyethylene, but tougher
higher softening temp
Naturally rigid by can be made flexible thru plasticisers
Polypropelene
Silimar e properties to P.ethy but is tougher
highter softening temp
Harder: used in thin-wall insulation
but less flexible, expensive
Nylon
Insect resistant outer jacket or sheath for underground use
Abrasion resistant
hard
smooth: difficult for insect/termite to grip
ABS
Can be formed into complex shapes
tough and shock resistant
- commonly used in computer and phone casings (injection molding process)
Polycarbonate
Similar mechanical properties to ABS
Good e insulation
heat resistant and flame retardant
Used in casings
Testing techniques
Multimeter
Megger test
Oscilloscopes
What needs to be tested (3)
Voltage
Current
Resistance
Test for inconsistencies and disruptions in circuit
Multimeter
Combines several instuments in one e.g. voltmeter, anmeter. ohmmeter, etc
Volt vs anm set up
Volt in parallel, anmeter in series
Insulation testing why
PRevent e shoks
ensure safety
reduct equiptment downtime
Megger testing
test for insulation faults: megohmmeter applied high voltage, measures resistance over time
high resistance = good, low = fault
Oscilloscope
Measure how signal changes over time
Can show:
Voltage/current changes over time
frequency of oscillating signal
Malfunctioning components
How much of signal is DC and AC
How much noise, whether noise is changing
Frequency Analysis
using a spectrum analyser
- use Fast Fourier Transform (FFT) to covert signals from time domain to frequency domain
Multimode cables
Short distance
High bandwidth support
higher cable cost
Lower electronics cost
Easier to terminate due to large core size
Single mode cable
Short and long distance
Highest bandwidth support
Lower cable cost
Higher electromics cost
Harder to terminate with smaller core size
Structure of Fibre OC
- high refractive index core
- Low r.i. cladding (ensures light signals stay trapped in core even with bending)
Advantages of FO
Lighter
Corrosion resistant
Wide band width + high transmission capacity (light faster then electricity
Problems of Fo
Attenuation: atomic absorption of light by glass
Scattering of light by flaws and impurities
Reflection of light by splices and connectors
DISPERSION:
Spreading/overlapping of light pulses with distance
caused by chromatic dispersion (different wavelengths, pink Floyd)
and modal dispersion
- fibres w large diameters: light travel along different modes/paths
- limits bit rate.
Multimode fibre types
allows alight to travel along many modes/paths
- Step index: high R.I core and low R.I cladding
- high attenuation and dispersion - Graded index: cores with higher R.I in the middle
- light travelling straight down travels slowly: reduces dispersion
